State of the Art of Alternative Thermal Waste Treatment Processes

Incineration is the globally predominant process for thermal waste treatment considering plant number and treatment capacity. About 255 million tons of waste is processed annually in about 2200 facilities. In addition to this well established treatment method, waste pyrolysis and waste gasification offer other thermal treatment methods. These so called alternative thermal treatment processes have recurrently been presented by different operators since the 1970s, using more or less creative names.They are typically characterized by a comparably complex technology. According to the providers, the advantages of these processes compared to incineration lie within higher electrical efficiencies and/or higher value of conversion products, for example the production of glazed and therefore non-elutable slags or the production of liquid energy sources.

Due to a number of set backs, these processes have not
gained importance in Germany; however, they have recently been rediscussed and
demanded abroud by some groups of interest and political decision-makers.
Proponents of the processes can meanwhile refer to long-term operation of facilities
in Asia, especially in Japan. However, entirely different frame conditions
apply in these countries compared to Europe or North America.

The already
mentioned practical experiences mainly refer to facilities for gasification and
pyrolysis. Among these classical alternative
thermochemical processes, other alternatives have been entering the market for
the last years. Plasma processes – implemented as plasma pyrolysis or plasma
gasification – realize the conversion of waste through contact with hot plasma
of at least 2,000 °C – partly ionized gas. According to the providers of these
technologies, this ensures low gaseous emissions and at the same time high
quality of conversion residues. Another alternative thermochemical process,
catalytic depolymerization, aims to convert solid residues into liquid hydrocarbons,
typically in one step and with the help of a catalyst. The products are supposed
to have fuel-like properties and can be used as a substitute for diesel. So
called HTC-processes (hydrothermal carbonization) are preferably used to treat
(wet) biowaste and sewage sludge. Residues are converted in an aqueous solution
into a carbonized product, which allows for optimized energetic and material
use.

Only little reliable operational experience is available for the currently
discussed plasma and depolymerization processes as well as processes for
hydrothermal carbonization, which are yet to be introduced into the market. In
some cases, not even plausible mass- and energy balances are available. For the
treatment of problematic by-products, only concepts exist, which have not been
field-tested yet.